Concrete lifting systems for lifting of concrete panels and other components typically involve the use of lifting anchors incorporated into the component during casting, with the head of the anchor being encased within a removable or disposable hollow void former to form within the surface of the component a recess within which the head of the anchor lies for releasable coupling to lifting equipment. In one commonly used construction technique concrete panels are cast on site on a concrete slab or other flat surface. To erect the panel it is lifted from a horizontal configuration which it is cast to a vertical configuration by tilting the panel about its lower edge and when in its vertical configuration it is then moved to the required position for installation while still suspended from the lifting equipment. The general construction method in which the panel is cast on site and then is initially lifted from the horizontal to the vertical in the manner described is commonly known as tilt-up construction. In tilt-up construction the panel can be lifted either using anchors in which their lifting heads are exposed to the upper face of the panel (this is known as face-lift) or to the upper edge of the panel (this is known as edge-lift). Face-lift and edge-lift anchors are of different construction and the choice between a face-lift and an edge-lift situation is determined by engineering considerations and structural considerations.
Concrete panels may also be pre-cast in a similar manner in a factory or other off-site facility. In that case the panel will usually incorporate edge-lift anchors by which, after casting, it is raised by the same type of tilt-up action to an upright configuration for storage and/or transportation to site at which it will be lifted from the truck by means of its edge-lift anchors while in its upright condition and then moved to the required position for installation while still suspended from the lifting equipment.
Accordingly, as used herein the term “tilt-up” is intended to include not only construction situations in which a concrete panel or other component is cast on-site and then raised by tilting to an upright configuration for installation but also the manufacture of pre-cast concrete panels and other components off-site in which the component after casting is raised by tilting to an upright configuration for storage and/or transportation to site for subsequent installation.
FIG. 1 shows diagrammatically an edge-lift anchor 1 in its installed position for tilt-up lifting. The anchor has a head 2 for coupling to lifting apparatus and an anchoring portion in the form of a pair of substantially parallel legs 4 extending from the head. The particular head 2 shown is designed for co-operation with a lifting clutch in the form of a ring clutch with an arcuate locking bolt received within the eye 6 of the head, although it is to be understood that the head could be of a different detailed design for use with other types of lifting apparatus. The legs 4 are profiled so as to lock into the surrounding concrete. The particular profiling shown forms the subject of patent application 2006201337 (the contents of which are hereby incorporated by reference) but it is to be understood that the legs 4 may have any other form of profile to lock into the surrounding concrete and the anchoring portion may even be of a form which does not use two parallel legs. At the commencement of tilt-up lifting, the anchor is subjected to a substantial shear force acting in the direction of the arrows shown in FIG. 2. In this configuration, the upper edge of the anchor lies quite close to the upper face of the panel and there is insufficient depth of embedment to resist the shear force. The current practice to resist that shear force so as to prevent the anchor from breaking through onto the upper face of the panel is to provide a shear bar 8 which passes across a recess in the upper edge of the anchor and is bent downwardly at each side of the anchor to form horizontal portions 8a embedded more deeply within the thickness of the panel and possibly associated with further reinforcing bars 10 as shown in FIG. 3.